Correction for issue1265 (pdb bug with "with" statement).

When an unfinished generator-iterator is garbage collected, PyEval_EvalFrameEx
is called with a GeneratorExit exception set.  This leads to funny results
if the sys.settrace function itself makes use of generators.
A visible effect is that the settrace function is reset to None.
Another is that the eventual "finally" block of the generator is not called.

It is necessary to save/restore the exception around the call to the trace
function.

This happens a lot with py3k: isinstance() of an ABCMeta instance runs
    def __instancecheck__(cls, instance):
        """Override for isinstance(instance, cls)."""
        return any(cls.__subclasscheck__(c)
                   for c in {instance.__class__, type(instance)})
which lets an opened generator expression each time it returns True.

Backport candidate, even if the case is less frequent in 2.5.

on each iteration.  I'm not positive this is the best way to handle
this.  I'm also not sure that there aren't other cases where
the lnotab is generated incorrectly.  It would be great if people
that use pdb or tracing could test heavily.

Also:
 * Remove dead/duplicated code that wasn't used/necessary
   because we already handled the docstring prior to entering the loop.
 * add some debugging code into the compiler (#if 0'd out).

Re-enable all the tests in test_trace.py except one.  Still not sure that these tests test what they used to test, but they pass.  One failing test seems to be caused by undocumented line number table behavior in Python 2.4.

hotshot.stats.load.

single-line loops.

and test_support.run_classtests() into run_unittest()
and use it wherever possible.

Also don't use "from test.test_support import ...", but
"from test import test_support" in a few spots.

From SF patch #662807.

[ 729622 ] line tracing hook errors

with massaging from me to integrate test into test suite.

[ 643835 ] Set Next Statement for Python debuggers

with a few tweaks by me: adding an unsigned or two, mentioning that
not all jumps are allowed in the doc for pdb, adding a NEWS item and
a note to whatsnew, and AuCTeX doing something cosmetic to libpdb.tex.

[ 631276 ] Exceptions raised by line trace function

It conflicted with the patches from Armin I just checked it, so I had
to so some bits by hand.

patch #617312, both on the trunk and the 22-maint branch.

Also added a test case, and ported the test_trace I wrote for HEAD
to 2.2.2 (with all those horrible extra 'line' events ;-).

This makes things a touch more like 2.2.  Read the comments in
Python/ceval.c for more details.

Use a slightly different strategy to determine when not to call the line
trace function.  This removes the need for the RETURN_NONE opcode, so
that's gone again.  Update docs and comments to match.

Thanks to Neal and Armin!

Also add a test suite.  This should have come with the original patch...